

Why would company A need to accomodate any other “app store” in their product, especially if one of their product’s selling point is how streamlined it is?
Why should Microsoft allow for other browsers to be installed on Windows? Why should Google allow for other search engines being selectable on Android and in Chrome? The reason in all these cases is the same: it is anti-competitive, and creates a monopoly. This results in unfairly high costs to users, where these users are 3rd party software developers or end users. Due to this countries have laws against this.
Companies obviously wouldn’t want to accommodate others in ways that cost them money, but that does not make it morally acceptable from a societal point of view.
The flute doesn’t make for a good example, as the end user can take it and modify it as they wish, including third party parts.
If we force it: It would be if the manufacturer made it such that all (even third party) parts for These flutes can only be distributed through their store, and they use this restriction to force any third party to comply with additional requirements.
The key problem is isn’t including third party parts, it is actively blocking the usage of third party parts, forcing additional rules (which affect existing markets, like payment processors) upon them, making use of control and market dominance to accomplish this.
The Microsoft case was, in my view, weaker than this case against Apple, but their significant market dominance in the desktop OS market made it such that it was deemed anti-competitive anyways. It probably did not help that web standards suffered greatly when MS was at the helm, and making a competive compatible browser was nigh impossible.